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tap making

In my recent attempts at tap making; I've come across a puzling problem which I suspect is due to faults in the heat treatment department (an old propane torch, firebrick and plastic bucket).

A correcty working tap, in use on brass, just sheared off, not near the top of the work which is what usually happens when I have trouble with blunt taps, but in the threaded portion well above the work. The shear looks like a sheared off allen key! My suspicion is that a crack developed there during hardening, since it happens to coincide with a damaged "tooth". I don't know how plausible this is, and I have been wondering if it could have been incorrect tempering.

The silver steel taps which are two fluted for use on brass, due to my not having any dividing gear are supposed to be hardened and tempered to dull yellow, which is easier said than done, but in this case the cutting part was yellow all over and the shank blue. It then went straight in the water and I wondered if the metal in the centre of the cutting portion would have had a chance to become hot enough to be tempered or if it would have remained dead-hard? Tap diameter is about 3/16".

Suggestions as to what I might have done wrong would be most welcome as this doesn't seem to be a subject anybody I know is knowledgeable about!

After trying this procedure a few times commercial ground thread taps seem very cheap!

Alan Bain

Reply to
Alan Bain
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Temper it by sticking it in a pre-heated oven at 175 Deg C for one hour per inch of thickness plus time to heat up. Do this straight after quenching. Don't quench after tempering.

Also try to avoid stress raisers on the tap before hardening. |t is permissible to completely grind/machine out damaged teeth before hardening so long as you don't tell anyone about it :-)

Mark Rand RTFM

Reply to
Mark Rand

Albeit not beeing the cause (or maybe?), I would make a three fluted tap. You don't need a dividing head, because the spacing between flutes is really not important. With two flutes (not spiral flutes), the danger of seizing is quite great (though it _might_ be the cause for another failure).

Nick

Reply to
Nick Mueller

Yet another thought. If you have to make a tap for a through hole or a taper tap for a deep hole, put a pilot on the end. It makes hand tapping so much simpler.

Mark Rand RTFM

Reply to
Mark Rand

The thread I was using was a bit non-standard (hence the need to make a tap) and I'd actually calculated a tapping drill for 80% engagement, but I'm aware that my ability to measure thread forms accurately is limited. The tapping was never hard (and from various economies in the past I've used blunt and very blunt BAS taps so I have a reasonable idea what these feel like).

Alan

Reply to
Alan Bain

In message , Alan Bain writes

How was the initial hardening done? Quenching in water especially for small taps can start cracks. Better off with saturated brine or oil.

Incidentally, I never gave 'brine quenching' much thought until doing it recently (on a tap) under a strong light. The brine solution forms a 'skin' of salt on the item (which regulates the cooling process) It then dissolves as the item cools. After 40 years you can still learn...

Reply to
Martin Akehurst

Alan, B&Q did (still do?) an offer of one of the high speed drills and accessories for about £20. The carborundom discs are ideal for sharpening taps, and the smaller diameter for dies.

In message , Alan Bain writes

Reply to
Martin Akehurst

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